Making paper from tetrafluoroethylene polymers



United States Patent 3,003,912 MAKING PAPER FROM TETRAFLUORO- ETHYLENEPOLYMERS Edward F. Harford, Wilmington, Del., assignor to E. I. du Pontde Nemours and Company, Wilmington, Del., a corporation of Delaware NoDrawing. Original application Apr. 27, 1954, Ser. No. 426,041. Dividedand this application Mar, 2, 1959, Ser. No. 796,219

2 Claims. (Cl. 162157) This invention relates to paper composed offibers of polytetrafiuoroethylene or other closely related polymer suchas polychlorotrifluoroethylene, tetraiiuoroethylenechlorotrifluoroethylene interpolymers, or telomers formed from one orboth of these monomers.

Paper has been made heretofore from rags, straw, bark, wood or otherfibrous material by the following essential steps: (1) reduction of theraw material to a thin pulp, (2) running this pulp upon a sieve of finemesh which retains the fibers which become felted together, and (3)removing and drying the felt so for-med. Most commonly, although notinvariably, the fibrous material employed in these operations has beenof a cellulosic nature. Paper made from non-cellulosic materials wasgenerally coarse textured and lacking in tensile strength or pliability.

There has, however, existed an important industrial need for paper whichwould not have the well-known disadvantages of cellulosic paper forcertain applications. Resistance to various chemicals, especiallycertain acids, has been one of the properties which has limited the useof cellulosic papers for clarification or filtering applications. Tomeet these requirements, industry has generally turned to such materialsas cloth (i.e. woven sheet materials) made of plastics or inorganicglasses. Such woven products are relatively expensive and it is'difficult to make them very thick. Since non-Woven sheets, andespecially sheets made by paper-making techniques, in general, have beenless expensive than woven sheets (cloth) it is apparent that importantopportunities have been awaiting the discovery of fibers which do nothave the disadvantages of cellulosic fibers, and which are suitable forpaper-formation in conventional paper-making machinery.

It has recently been discovered (Llewellyn, U.S.

2,57 8,529) that upon repeatedly passing particles ofpolytetrafiuoroethylene through milling rolls, a compacted mass isformed, and this compacted mass, upon repeated rerolling, changes inphysical appearance, and assumes the form of matted shreds, flakes orstrands of the polymer. The resulting matted material has acharacteristic toughness and resilience which make it especiallyvaluable for use as a gasket or packing material. In a somewhat relatedprocess (Edgar, U.S. 2,578,522) the curds obtained by coagulating anaqueous suspensoid of polytetrafiuoroethylene are rolled to produce aself-supporting film, which can be converted to a smooth, denser productby calendering on pressure rolls. There have been still other processeswherein polytetrafiuoroethylene in finely divided powdery form has beencompressed into film (U.S. 2,406,127; 2,520,173; 2,400,099). Moreover,it has very recently been :found possible to extrudepolytetrafluoroethylene in the form of a film by using particles ofcolloidal size in combination with an organic thickener. The latterprocess also has been found to be efiective for extrusion ofpolytetrafiuoroethylene fibers and filaments. However, none of theprocesses hereinabove mentioned can be regarded as producingpolytetrafluoroethylene paper; for example, the rolled matted shreds ofLlewellyn are not a form of paper, defined in the term of the 3,003,912Patented Oct. 10, 1961 ice essential steps hereinabove set forth, andthe impervious sheets made of compressed or extruded particles not onlyfail to meet this definition, but also are different in that they failto exhibit properties which are characteristic of felted fibers. Forexample, they do not adhere strongly to each other even after heatingbeyond the transition point of 327 C. It has been discovered inaccordance with this invention that tetrafiuoroethylene polymers andpolymers closely related thereto, in fibrous form, can be converted topaper by the application of the steps which are essential topaper-making, in processes which are well known in the art.

The polymer employed may be either polytetrafiuoroethylene,polychlorotrifiuoroethylene, tetrafluoroethylenechlorotrifiuoroethyleneinterpolymer, or any of the said polymers with end groups supplied bytelomer-forming react-ants such as methanol, isopropanol, etc.

-It has been discovered also that paper can be made, by essentialpaper-making operations, from said polymers in fibrous form admixed withglass fibers.

in a specific embodiment, this invention can be practiced by extruding asuspension or paste of polytetrafiuoroethylene in the form of afilament, rod, or tube, and cutting the resulting material into smalllengths, followed by mechanically working the resulting pieces toproduce fine fibers of much smaller diameter than the said filament,rod, or tube (as hereinbelow explained), pulping the said fibers,running the pulp upon a fine mesh sieve whereby the fibers becomefelted, and removing and drying the felt so formed. Aside from theseessential operations other important modifications may also be included.For example, the sheet can be sintered, suitable at 350-370 C., whichgreatly improves the strength of the paper without destroying its airpermeability-.

The paper thus formed resembles ordinary paper in pliability,permeability, and strength, but is very much superior to ordinary paperin heat-resistance and resistance to chemicals. It is non-inflammable,and non-hydrolyzable. In particular applications this novel paper can bebonded to an impervious ply of polytetrafluoroethylene. Such sheetscomprising polytetrafiuoroet'hylene paper bonded by sintering to animpervious polytetrafluoroethylene base can readily be bonded to othersurfaces, since the paper face can be adhesively united with cloth,

metal surfaces, glass surface, etc., through the use of commonadhesives, even including adhesives which are known to be ineltectivefor bonding previously known polytetrafluoroethylene films to othermaterials.

The organosols or aqueous pastes which can be employed in themanufacture of fibrous tetrafluoroethylene suitable for use in themanufacture of paper by the essential paper-making operations are wellknown. A common characteristic of all of these fiber-forming mixtures isthe colloidal size of the polytetra-fluoroethylene particles containedtherein. These colloidal particles, in particular embodiments, may beagglomerated into somewhat larger masses provided the colloidal surfacesremain intact.

Example 1.--A dispersion containing 25 grams of poly-tetrafluoroethylenein 40 grams of water was coagul-at'ed by mechanical beating (U.S.2,593,583), and the resulting coagulated colloidal product was separatedfrom the water and dried. The dry powder was lubricated with a quantityof Skellysol-ve, E (petroleum fraction) suffiacteristic internalphysical structure of the original rod as initially extruded. These finefibers had the capacity for matting together to such an extent that theywere suitable for conversion to paper in standard paper-makingmachinery. Upon sintering at 360 the sheets shrank to 41% of theirinitial area, Similar sheets were made by admixing thepolytetrafluoroethylene fibers, prior to paper-making, with equalamounts of glass fibers, rock Wool fibers, asbestos fibers, and flakemica, respectively. In the absence of sintering the paper thus obtainedwas in each instance soft and of low strength; after sintering it becamemore dense and had fair tear strength. A specimen of paper made in thismanner from 20 parts by weight of polytetrafluoroethylene and 8 parts byweight of asbestos, pressed hot enough to sinter, had a Mullen burststrength of 58 lbs. and a Gurley Densometer value of 14 sec.

Generally speaking, it is preferable that the extrusion die be smallenough so that the filament initially obtained will have the orientedstructure just described, and this is readily achieved by using dies ofabout one eighth inch, or so, in diameter. The diameter of the filamentcan be much narrower, or somewhat wider, than one eighth inch, dependingin part on the method used for transferring the filament to the cutter.Moreover, there is no requirement that the individual fibers be all ofthe same diameter or length. The relatively short lengths of filamentcan be broken up, i.e. converted to fine fibers, in a micropulverizerhammer mill if desired. It is sometimes helpful to pass the materialtwice through the micropulverizer or to separate the relatively coarsematerial and recycle the coarse fibers so as to produce fibers ofsuitable size for pulping.

In the preparation of the pulp a wetting agent may be added if desired.For example, a suitable wetting agent is Triton X-100, which is alkylaryl polyethylene glycol. In general no difiiculty is experienced in thepreparation of the pulp if the fibers are in lengths of about A in, to 1in. A beater may be used in the conventional manner, familiar to thepaper manufacturing art.

The pulp concentration should be relatively dilute, a suitableconcentration being in the range of about 5 to 20% preferably 8 to 10%by weight. Water is a suitable pulping medium although other liquidmedium may be employed if desired. It is important to avoid any undueexcess of wetting agent since this causes the pulp to sink. On the otherhand, if not enough wetting agent is employed it is relatively ditficultto wet the fibers and as a result the fibers tend to float on thesurface of the water. Anti-foaming agents may be employed with the pulpas desired, to prevent excessive foaming.

The pulp prepared as above described is suitable for use in standardpaper-making machinery.

In a series of tests weighed quantities of the pulp were placed in thehead box of a laboratory paper-making machine, and converted to sheetswhich measured 8 inches by 8 inches. Thirty grams of pulp (dry basis)produced sheets having a thickness of about 50 mils. In tests with thisquantity of pulp 2 to 3 gallons of water were used in the head box asthe liquid medium. After the mat was formed, water being drained oil? inthe usual fashion, the sheet was calendered and then removed from theFourdrinier wires. Uniform sheets which had enough strength to beremoved in the usual manner were thus obtained. It was found thatcalendering could be continued until the sheets became almostimpervious, but in general, the calendering was not continued to thisextent. The porous sheets thus obtained were dried and sintered in anair oven at 350-370 C. which caused the fibers to become self bonded. Italso caused shrinking of the sheet to about 40% of its area prior todrying. The air permeability of several sheets thus formed were measuredby the ASTM method D-737. The results were as set forth in the followingtable:

TABLE I Air permeability of tetrafluoroethylene resuz paper AirPermeability, Sheet cubic feet-minute Sample Weight, per sq. It. of areaDescription 01 Fiber Pulp No. oz./sq. yd. at a pressure ditferentlal 010.5 in. of water 1 51 0.05 Fine, short fibers (sheet calendered). 25 5Fine, short fibers. 51 10 Same as 2. 46 20 Coarse, long fibers. 25Medium diameter and length. 18 50 Same as 2. 25 73 Same as 3.

Tensile strength measurements were also made on sheets prepared as abovedescribed (TAPPI Method T404, at 23 C.). In a typical case with aspecimen about /2 in. wide (0.528 in.), the distance between jaws being2 in., the maximum load was 11.5 lbs. at an ultimate elongation of 88%.This particular specimen had a thickness of 0.057 in. The weight of thespecimen was 31 oz. per sq. yd.

The procedure hereinabove described is suitable for use in themanufacture of paper having said thicknesses as low as about 0.02 in.(20 mils). In general the products thus obtained at thicknesses of about0.025 to 0.10 inch have air permeability of about 5 to 75 cubic ft./min./ sq. ft. of area at a pressure dilferential of 0.5 in, of wateralthough, of course, continued calendering before heating beyond thetransition point produces sheets which are less pervious to air asexplained above. Paper thus obtained is further characterized in that itdoes not break when a /2 in. strip thereof is subjected to a load of 10lbs/0.05 sq. in. of thickness.

While the method of this invention has been hereinabove described asbeing applicable to the manufacture of relatively thin pliable sheets ofpaper (0.025 to 0.10 inch), it is to be understood that much thickersheets can be made by following substantially the same procedure. Infact, it is noteworthy that polytetrafluoroethylene felt at the time ofits formation is sufliciently porous so that relatively thick felts canbe obtained i.e. felts having a thickness of 2 to about 2V2 in. or more.

The invention may be used in the manufacture of paper composed of amixture of polytetrafluoroethylene fibers and other fibrous materialsuch as asbestos, glass, rock wool, flake mica, etc. For example, a pulpcomposed of 67% fibrous polytetrafluoroethylene and 33% of A; in. longglass fibers, prepared in the above-described manner became firmlybonded in the sheet and could be processed in ordinary paper-makingequipment to produce a paper composed of the saidpolytetrafluoroethylene fibers and glass fibers.

It is to be understood that all the polymers hereinabove disclosed asbeing suitable for use in the practice of this invention may beconverted to paper by the method hereinabove illustrated with referenceto polytetrafluoroethylene.

The word paper, as employed herein has its conventional meaning, andthis in itself establishes the structural or spacial disposition of thefibers with respect to each other. The fibers are in intimatemulti-contact relation ship, and are interlaced in a random manner,being compressed in a way which reflects the effect of the calenderingstep. A remarkable inherent feature of the paper composed ofpolytetrafluoroethylene fibers is the shrinking which attends air-dryingabove the crystalline melting point, unless mechanical devices areemployed to prevent such shrinking. The shrinking results in moreintimate contact between the fibers and in efiect produces more knottingtogether, with attendant increase in strength, without loss of airpermeability as noted hereinabove. It is characteristic of the hereindisclosed polytetrafluoroethylene fiber that it retains its fibrous formeven when heated above its crystalline melting point, because of thephenomenally high viscosity of the resin at the temperatures involved.

This application is a division of application Serial No. 426,041, filedApril 27, 1954, and now abandoned.

The invention is limited only as set forth in the following claims.

I claim:

1. A process for forming a compressed air-pervious sheet of mattedfibers which comprises extruding lubricated colloidal particles ofpolytetrafluoroethylene in the form of a filament, cutting the saidfilament into lengths of predominantly from one-quarter inch to oneinch, subjecting the resulting lengths of filament to a rubbing actionwhereby it shreds into fibers of smaller diameter than that of theextruded filament, pulping the said fibers, running the resulting pulpupon a sieve of fine enough mesh to retain the said fibers, whereby thefibers become felted together, removing, calendering, and drying thefelt so formed, and sintering the resulting sheet sufiiciently tostrengthen the paper thus produced, Without rendering it impervious toair.

2. An air-pervious sheet comprising bonded polytetrafluoroethylenefibers which are randomly interlaced in multi-contact relationshipwherein the binding consists essentially of sinteredpolytetrafluoroethylene.

References Cited in the file of this patent UNITED STATES PATENTSWooding Oct. 22, 1957

2. AN AIR-PERVIOUS SHEET COMPRISING BONDED POLYTETRAFLUOROETHYLENEFIBERS WHICH ARE RANDOMLY INTERLACED IN MULTI-CONTACT RELATIONSHIPWHEREIN THE BINDING CONSISTS ESSENTIALLY OF SINTEREDPOLYTETRAFLUOROETHYLENE.